J. Org. Chem. 1993,58,6654-6661
6654
Conformational Studies of Dihydrotetraphenylmethanes. 2. X-ray Crystallographic and Solution ‘HNMR Studies of cis-1,4-Dihydro-4-tritylbiphenyland Its 4’-Bromo Derivative: Conformational Control by an Intramolecular Edge-to-Face Aromatic Interaction Martin C. Grossel,**+ Anthony K. CheethamJJ David A. 0. Hope? and Simon C. Westont Chemistry Department, University of Southumpton, Highfield, Southampton, SO9 5NH1 U.K.,and University of Oxford, Chemical Crystallography Laboratory, 9 Parks Road, Oxford OX1 3PD, U.K. Received August 6, 1993.
X-ray structural studies of cis-1,4-dihydro-4-tritylbiphenyl(2a) and ita 4’-bromo derivative 2b reveal that, in contrast with earlier predictions based on solution NMR data, both adopt a similar congested conformation in which the cyclohexa-l,li-diene ring is almost planar, forcing the isolated aromatic ring into the r-cloud of one of the benzene rings of the trityl substituent and leading to considerable stretching of the C1-C11bond and a close intramolecular edge-to-face aromatic interaction. Variabletemperature lH NMR experiments suggest that this interaction becomes increasingly important at low temperatures. Crystals of cis-l,4-dihydro-4-tritylbiphenyl(ta)are triclinic, space group Pi, 2 = 2, lattice parameters a = 9.566(2) A, b = 10.125(1) A, c = 11.924(2) A, a = 79.51(1), 6 = 75.56(1)O, y = 82.44(1)O. 1678 independent reflections gave a final R of 0.041. Crystals of 4’-bromo-cis-1,4dihydro-4-tritylbiphenyl(2b)are orthorhombic, space group P212121,Z = 4, lattice parameters a = 10.036(6) A, b = 15.785(6) A, c = 17.209(9) A. 1709 independent reflections gave a final R of 0.049.
wve
Introduction While the conformationalanalysis of biphenyls is now well understood,’ the behavior of the 1,4-dihydrobiphenyls has received much less attention. We have been particularly interested in the solid-state and solution conformational behavior of some sterically congested derivatives, particularly the stereoisomeric dihydrotritylbiphenyls 1 and 2. For example, we have reported2 the results of an X-ray crystallographic study of trans-1,4-dihydro-4-tritylbiphenyl (la)and two crystalline modifications of ita 4’-bromo derivative lb. A comparison with solution lH NMR data for these compounds led us to conclude that, contrary to earlier suggestions: the cyclohexa-1,4-diene ring is almost planar in the lowest energy conformations of this structure. Furthermore, in the solid state, the trityl group appears to lock the geometry of one end of the dihydroaromatic ring, the precise nature of the favored (boat) conformation of the latter being determined by crystal packing requirements. Cyclohexa-l,4-diene (3) itself is thought to occupy a very shallow vibrational potential energy well in solution with a single energy minimum corresponding to a planar and further support geometry (a = 180’ in Figure 1),4-6
* To whom correspondence should be addressed. + University of
Southampton. University of Oxford. I Current address: Materials Department, University of California, Sank Barbara, CA 93106. Abstract published in Advance ACS Abstracts, October 15, 1993. (1) Thispaperisatributetoand afinalpublicationfromtheChemistry Department of Bedford College, University of London (1872-1986), where part of thin study was carried out and where much key work on biphenyl stereoisomerismand clathrate formation was undertaken; eg., see: Hall, D. M.; Turner, E. E. J. Chem. SOC.1956,1242. Comments in: Weber, E.; Czugler, M. In Topics in Current Chemistry; Weber, E., Ed.; Springer-Verlag: Berlin, Heidelberg, 1988; Vol. 149, p 47. (2) Cheetham,A. K.;Grosse1,M. C.;Newsam, J. M. J.Am. Chem.SOC. 1981,103,6363. It should be noted that the crystallographic a- and c-axis parameters for la should be reversed. (3) Durham,L. J.;Studebaker,J.; Perkins, M. J. J.Chem. SOC.,Chem. Commun. 1968,456. Atkinson, D. J.; Perkins, M. J. Tetrahedron Lett. 1969,2335. 8
%
I
2
3
Figure 1. Puckering angle a and pseudoaxial (9.)and pseudoequatorial (9.)substituent locations within a 1,4-dihydrobenzene boat conformation.
H I a,Ar=Ph b, Ar =
p&cSH1
2a,Ar=Ph b, Ar = p-BC6H4
H H 3
4
for this view comes from a recent X-ray structural study of this molecule at 153 K.7 However, both infrared6 and NMR spectroscopy suggest that several vibrational energy levels corresponding to a boat-boat flexing about a planar energy minimum, e g . , 4, are significantly populated at room temperatureeBMolecular mechanics and ab initio molecular orbital calculations have supported this pro~ o s a l .So ~ far we have been unable to ascertain whether a similar situation (i.e., a single minimum potential well) (4) Rabideau, P. W. In Conformational Analysis of Cyclohexenecl, Cyclohexadienes, and related Hydroaromatic Compounds; Rabideau, P. W., Ed.;VCH Publishers Inc.: New York, 1989; Chapter 4. (6)Laane, R.; Lord, R. C. J. Mol. Spectrosc. 1971, 39,340. (6) Groesel, M. C.; Perkine, M. J. Nouu.J. Chem. 1979, 3, 286. (7) Jeffrey, G. A.; Buschmann, J.; Lehmann, C. W.; Luger, P. J.Am. Chem. SOC. 1988,110,7218.
0022-3263/93/1958-6654%04.00/00 1993 American Chemical Society
J. Org. Chem., Val. 58, No. 24, 1993 6666
Conformational Studies of Dihydrotetraphenylmethanes
pertains for the trans-l,4-dihydro-4-tritylbiphenyls 1,with crystal lattice requirements distorting the shape of the potential energy well and shifting the position of the
minimum, or whether the structure favors a double minimum potential energysurface with rapid equilibration i n solution between relatively inverted boat conformations.2 We have now completed X-ray crystallographic and solution 1H NMR studies of the cis isomers Zagand 2b. Solution 1H NMR data for 2a have previously been interpreted as indicating a grossly puckered boat geometry (a 1 6 5 O ) in which the substituents are locked pseudoequatorially Wein Figure l).4JO Our results shed further light on the conformational potential energy surface for the 1,4-dihydrotetraphenylmethanesin solution and in solid state and provide a unique opportunity for a detailed comparison of t h e solid-state behavior of a group of closely related structures. In addition, we discover that both cis isomers 2 a d o p t conformations i n which there is a remarkably close intramolecular edge-to-face aromatic interaction. Such phenomena are now known to play an important role in protein folding" and molecular recognition.12
Table I. Summary of Crystal Data, Intensity Collection, and Data Processing compd mol form. M,, g mol-' cryat shape cryst size, mm color space grp cryst syst a, A
b, A
c, A a,deg
8, deg
yt deg
u,A3 z
Doh, g cm-' D d C ,g cm-' abs correctn radiation A,
A
scan mode T,K 26 limita, deg no. of data used, I>3 4 weights13 R, % Rw, %
Experimental Section Crystal Data. Crystallographic data for 2a and 2b are compared in Table I. (a) cis-1,4-Dihydr0-4-tritylbiphenyl(2a): C!&=, M, = 398, triclinic Pi,a = 9.566(2) A, b = 10.125(1) A, c = 11.924(2) A, a = 79.15(1)', B = 75.56(1)', y = 82.44(1)", U 1094.13 As, D0b = 1.21 g cm-3, D d c = 1.18g cm-9 for Z = 2. Mo Ka,X = 0.710 69
A.
Preliminary photographs of a good quality crystal (0.2 X 0.2 0.3 mm) indicated that it had triclinic symmetry. Intensity data collection was then carried out at room temperature on an Enraf-Nonius CAD-IF diffra~tometer.'~ Using an w-28 scanning mode, data were collected automatically for half the limiting sphere in the range 8 = 0-25O. No correction for absorption was found to be necessary. Lorentz and polarization corrections were applied, and the data were scaled, sorted, and merged to give 1678 independent structure amplitudes with Z > 30, where Z is the final measured intensity and u is the standard deviation derived from the counting statistics. X
The structure was solved in space group PI by direct methods using the program MULTAN 77." A block factor approximation to the full least-squares matrix was employed during the refinement of the positional and thermal parameters of the carbon atoms. All the hydrogen atoms were located during a difference Fourier synthesis. In many cases the definition was poor, making it necessary to place the hydrogen atoms geometricallyafter each cycle of refinement of the carbon atoms. During the final cycles (8)Lipkowitz, K.B.; Rabideau, P. W.; Raber, D. J.; Hardee, L. E.; Schleyer,P. v. R.; Kos, A. J.; Kahn, R.A. J. Org. Chem. 1982,47,1002. (9) For a preliminaryreport on the structure of 3a see: Grosael, M. C.; Cheetham, A. K.; Hope, D. A. 0.; Lam,K. P.; Perkins, M. J. Tetrahedron Lett. 1978, 5229. (10) Rabideau, P. W. Acc. Chem. Res. 1978,11,141. Rabideau, P. W.; Paschal,J. W.; Patterson, L. E. J. Am. Chem. SOC.1975,97,5700. (11)Burley, 5.K.;Petako, G. A. Science 1985,229, 23. (12) Seel,C.; V w e , F. Angew. Chem., Int. Ed. Engl. 1992,31,52& 549 and references cited therein. (13) Enraf-Nonius CAD-IF Diffractometer User Handbook; Enraf-Nonius: Delft, Holland, 1977. (14) Gennain, G.; Main, P.; Woolfson, M. M. Acta Crystallogr. 1971, A27,368.
2b.CsCs
2a
CsiHm 398 irregular 0.2 X 0.2 X 0.3 colorless
Pi triclinic 9.566(2) 10.125(1) 11.924(2) 79.15(1) 75.56(1) 82.U(1) 1094.13 2 1.21
1.18 none Mo Ka! 0.710 69 w-2e 298 1-25 1678 3-term Chebyshevls 4.08 5.08
CS~HSIB~ 556 needle 0.1 X 0.1 X 0.3 colorless P212121 orthorhombic 10.036(6) 15.785(6) 17.209(9) 90 90 90 2726.6 4 1.30 1.32 (includes benzene) none Mo Ka! 0.710 69 w-26
143 1-25 1709 3-term Chebyshevls 4.90 5.80
of refinement, a three-term Chebyshev series was used in place of the unit weighting scheme.lS A final R value of 4.08% (R, = 5.08%) was obtained. (b) 4'-Bromo-ch-1,4-dihydro-4-tritylbiphenyl(2b):CU&IBr, M, = 556, orthorhombic, space group P212121, a = 10.036(6) A, b = 15.785(6) A, c = 17.209(9) A, a = 6 = y = 90', U = 2726.6 A3, Z = 4, D0b = 1.3 g cm-9, D d c = 1.32 g cm-9. Mo Ka,X = 0.710 69 A. Weissenberg photographs displayed the systematic absences h00 ( h = 2n), OkO (k = 2n), indicating the space group p212121 or P21212. These were distinguished later by diffractometry; a scan of the h00, OkO,and 001 reflections revealed that all those with an odd index were missing, confirming the orthorhombic space group P212121. A discrepancy was noted between the calculated and observed densities ( D d c= 1.16 g cma, Doh = 1.3 g cm-3). This was caused by the presence of four molecules of benzene in the unit cell, which were subsequently located by a difference Fourier synthesis. A temperature of -130 'C was maintained during the data collection, which was carried out as before. 1709 independent reflections were obtained after correction, scaling, and merging. Patterson methods were employed in the solution, and the refinement followed a course similar to that of cis-l,l-dihydro4-tritylbiphenyl(2a), the benzene molecule being constrained to be a rigid body, to give a final R value of 4.90% (R, = 5.80%). Atomic coordinates for the non-hydrogen atoms of 2a and 2b are listed in Table 11,and selected bond lengths and bond angles for the two structures are compared in Tables I11 and IV respectively. The numbering scheme used for these data is shown in Figure 2. Atomic coordinates of hydrogen atoms and thermal parameters for both structures have been deposited with the Cambridge Crystallographic Data Centre." NMR Experiments. Spectra were recorded on Perkin-Elmer R24B and Bruker WH360 spectrometers as solutions in CDaCl2, CDCls and DMSO-de. Assignments and interproton coupling constanta were determined by selective 'H-lH decoupling and COSY experiments.
Results and Discussion X - r a y C r y s t a l l o g r a p h i c Results. The solid-state geometries of cis-1,4-dihydro-4-tritylbiphenyl(2a) and its (15) Computing Methods in Crystallography; Rollett, J. S., Ed.; Pergamon Press: Oxford, 1965; p 40.
6656 J. Org. Chem., Vol. 58, No.24, 1993
Grossel et al.
Table 11. Atomic Coordinates for Non-Hydrogen Atoms in 2 atom
xla
rib
ZIC
(a) cis-1,4-Dihydro-4-tritylbiphenyl(2a) c1 0.3459(3) 0.2207(3) 0.6493(3) c2 0.4293(4) 0.0904(3) 0.6886(3) c3 0.5258(3) 0.0786(3) 0.7528(3) c4 0.5639(3) o.ig~(3) 0.8001(3) c5 0.4896(3) 0.3254(3) 0.7531(3) C6 0.3929(3) 0.3374(3) 0.6888(3) c7 0.7330(3) 0.2020(3) 0.7782(2) c11 0.3627(3) 0.2458(3) 0.5169(3) c21 0.4977(4) 0.2692(4) 0.4433(3) C31 0.5175(4) 0.2911(4) 0.3231(3) C41 0.4019(4) 0.2902(4) 0.2739(3) C51 0.2672(4) 0.2671(4) 0.3451(4) C61 0.2472(4) 0.2448(4) 0.4659(3) C80 0.7908(3) 0.2288(3) 0.6445(2) C90 0.8015(3) 0.3592(3) 0.5808(3) Cloo 0.8485(4) 0.3804(3) 0.4595(3) CllO 0.8816(4) 0.2725(4) 0.3989(3) c120 0.8665(4) 0.1433(3) 0.4599(3) C130 0.8226(3) 0.1221(3) 0.5811(3) C81 0.8090(3) 0.0718(3) 0.8356(2) c91 0.7402(4) -0.0102(3) 0.9359(3) ClOl 0.8155(4) -0.1194(3) 0.9902(3) Clll 0.9611(4) -0.1494(3) 0.9473(3) c121 1.0319(4) -0.0682(3) 0.8488(3) C131 0.9569(3) 0.0409(3) 0.7940(3) C82 0.7644(3) 0.3136(3) 0.8391(3) C92 0.6702(4) 0.3507(3) 0.9399(3) c102 0.7033(4) 0.4461(4) 0.9968(3) c112 0.8308(5) 0.5061(3) 0.9543(4) c122 0.9285(4) 0.4673(3) 0.8567(3) C132 0.8953(4) 0.3720(3) 0.8008(3) (b) 4’-Bromo-cis-1.4-dih~dro-4-trit~lbi~henvl(2b) Brl 0.7250(1). 0.1761(7) 0.6996(9) 0.4733(5) c1 0.2120(7) 0.4327(6) 0.5816(11) c2 0.2576(6) 0.3670(5) c3 0.5839(9) 0.2718(6) 0.3208(5) c4 0.7092(9) 0.2373(6) 0.3626(6) c5 0.8304(9) 0.1931(6) 0.8261(10) 0.42816) C6 0.36996) 0.2970(5) c7 0.7287(9) 0.2102(6) 0.5563(5) 0.7069(9) c11 0.2952(6) 0.7075(10) 0.5690(5) c21 0.3279(6) 0.6444(5) 0.7131(10) C31 0.2723(6) 0.7175(12) 0.7066(4) C41 0.1868(6) 0.7150(10) 0.6956(5) C51 0.1548(6) 0.7084(10) 0.6199(5) C61 0.4206(5) 0.7393(9) 0.3727(5) C80 0.4560(7) 0.4067(6) 0.6237(9) C90 0.5026(7) 0.6339(11) 0.4759(6) Cloo 0.5124(6) 0.5119(5) 0.7560(15) CllO 0.4750(7) 0.4791(5) 0.8691(11) c120 0.4291(6) 0.8600(10) 0.4103(6) (2130 0.3995(6) 0.6120(8) 0.2450(5) C81 0.3453(6) 0.2035(5) 0.5226(10) c91 0.3774(8) 0.4320(11) 0.1527(6) ClOl 0.4637(7) 0.1441(6) 0.4133(10) Clll 0.5189(6) 0.1825(6) 0.4985(8) c121 0.4879(6) 0.2343(6) 0.5958(12) C131 0.3829(7) 0.2431(6) 0.8548(11) C82 0.4608(6) 0.9157(13) 0.2401(6) C92 0.4771(7) 0.1872(7) c102 1.0153(9) 0.4145(7) 0.1339(6) c112 1.0502(11) 0.3379(6) 0.9886(11) 0.1352(6) c122 0.3215(7) 0.8854(8) 0.1889(6) C132 0.1860 0.7461 0.0313 C140 0.1402 0.8589 0.0565 (2141 C142 0.0894 0.8511 0.1198 0.7357 0.1605 0.0818 C143 (2144 0.6202 0.1378 0.1245 0.6261 0.1785 C145 0.0719
4’-bromo derivative 2b are shown in Figures 3 and 4, respectively. Comparison with the structures of the stereoisomers la and l b (Figure 5) immediately reveals
Table 111. Selected Bond Lengths (A) with eid’i in Parentheres bond 2a 2b 1.487(14) 1.512(13) 1.529(12) 1.340(14) 1.505(13) 1.514(13) 1.613(11) 1.327(13) 1.534(12) 1.547(12) 1.582(14) Table IV. Selected Bond Angles (de& with eid’i in Parentheses 2a 2b (a) Bond Angles Involving Non-Hydrogen Atoms 110.6(3) 111.1(7) 110.0i8) 112.2(8) 125.9(10) 122.7(9) 111.5(7) 112.3(7) 111.6(8) 124.5(9) 123.8(9) 119.9(8) 120.8(8) 117.1(9) 118.1(8) 107.5(8) 118.6(10) 116.9(11) 104.4(8) 118.6(9) 117.8(8) 106.1(8) 118.0(9) 106.8(9) 117.7(9) 110.6(7) 108.9(7) 0 Angles about the cyuhexadiene ring. (Note that C-H bond lengths were constrained to 1.00(1) A.)
41
51
f i 31
1 01
12
i1i
110
Figure 2. Numbering scheme used for the crystallographic data.
what is perhaps the single most important (and remarkable) observation in this work, namely that the gross features of all five structures are almost identical, despite the change in the relative dispositions of the phenyl and trityl groups. The conformation of the cyclohexadiene boat remains close to planar in all five structures with ol16 lying in the range f1’72O to 180°, and this ring simply behaves as a structural hinge between the aryl and trityl groups.
Conformational Studies of Dihydrotetraphenylmethanes
J. Org. Chem., Vol. 58,No.24, 1993 6657
9 Ph
a +ve
u -ve
Figure 6. Effect of boat-boat inversion in the cis-dihydrotritylbiphenyls 2.
"i, Figure 3. General view of the asymmetric unit of 2a.
P
Figure 4. General view of the asymmetric unit of 2b,showing the location of the benzene of solvation.
Y
I I
I I
r" Figure 5. Overlay comparing the solid-state conformations of the tram- and cis-l,4-dihydro-4-tritylbiphenylsl2 and 2 (2a in bold). Both cis isomers 2 adopt similar conformationsin which the Substituents are placed pseudoequatorially. In the parent hydrocarbon 2a,the cyclohexadiene ring has amem = -175O but is slightly distorted being more puckered at C-4, the carbon atom bearing the trityl substituent. A
similar situation is found in 2b, but in this case the = -178O. In cyclohexadiene ring is almost planar, ,a this latter structure the asymmetric unit also contains one molecule of benzene, the crystallizing solvent." It should also be noted that data from the crystal of 2b were collected a t -130 "C. For both cis isomers 2 the isolated phenyl substituent is oriented along the C(l)-C(4) axis of the dihydroaromatic ring in a manner similar to that found in each of the trans structures 1.2 However, this has important implications for the cis derivatives since when the cyclohexadiene ring is relatively planar, the ortho proton, H21 in 2 (see Figure 6),is forced into the ?r-cloud of one of the phenyl rings of the trityl substituent (see Figures 3 and 4). Indeed, in the parent hydrocarbon 2a the proton H21 is located only 2.55 A above the best plane of the remote benzene ring, while for 2b this separation is 2.48 A.18 This feature is particularly significant in explaining unusual aspects of the lH NMR spectra of solutions of 2a and 2b (see below). As the cyclohexadiene ring conformation is changed from a planar to a more puckered geometry in which the substituents are located pseudoequatorially, the distance between the proton H21and the ?r-cloudis increased (Figure 6). The solid-state conformation of the bromo derivative 2b (where a = -178') forces the hydrogen atom Hzl and the benzene ring to lie within the sum of their van der Waals radii (2.9 A) (though it is not clear that a typical benzene radius of 1.7 A is appropriate for the center of an aromatic torus). Inversion of the ring conformation so that the substituents are placed pseudoaxial exacerbates this problem, and it would seem that the solid-state conformations of 2 must approach one limit for conformational flexingof the cis-dihydrotritylbiphenylskeleton. It should also be noted that the phenyl and trityl substituents adopt essentially identical orientations relative to the cyclohexadiene ring in both the trans-isomers 1 and the cis-isomers 2 (see Figure 5).299 A more detailed description of the cyclohexadiene ring geometry is conveniently provided using the torsion angles and &, in Figure 7. These indicate the positions of the ~~~
~
(16) The angle a is that between the best planes defined by C l + C& and C & , - C & in Figure 1; it is positive when the P4C- group is pseudoaxial. Figure 1 also shows the pseudonxial, q., and pseudoequatorial, qe,positions in a boat cyclohexa-l,4diene ring. (17) The presence of one molecule of benezene per molecule of 2b in the crystal is alsoevident from the relative integration valuea for aromatic and other reasonances in solution 1H NMR spectra of this compound. (18) If this benzene ring is regarded as the xy plane, then the proton H(21) lies at x = 0.30, y = 0.00,z 2.55 A and C(21) at x = 0 . 6 4 , ~= 0.10, z = 3.49 A with respect to ita centroid in 2a; with H(21) at x = 0.05, y = 0.04, t = 2.48 A and C(61) at x = 0.33, y = 0.32, z 3.48 A in 2b,baeed on a C-H bond length of 1.0 k (19) Calculations were performed using the CRYSTALS suite of programs: Watkin, D. J.; Carruthers, J. R.; Betteridge, P. W. CRYSTALS User Guide;Chemical Crystallography Laboratory, Oxford University: Oxford, U.K. Figures were drawn using Chem-X (ChemicalDesign Ltd., Oxford, U.K.).
-
-
6668 J. Org. Chem., Vol. 58, No.24, 1993
Grossel et al.
&.& N H H
@e
Figure 7. Torsion angles about a cyclohexa-1,4-dienering. Table V. Comparison of Key Structural Data for the trans- and cis-1,4-Dihydr0-4-tritylbiphenyls1 and 2, Reswctively angle (de / distance
,“A)
la
lb lb monoclinic orthorhombic
2a
2b
(a) Dihydrobenzene Ring Geometry (Average Values)# 172 -173 -176 -175 -178 75.2 68.2 60.7 42.3 74.0 B[Hl]b 50.3 54.0 74.1 72.3 B[H41b 81.5 118.1 120.3 126.3 121.3 +[Clllb 129.6 133.7 127.4 132.9 132.4 +[C7Ib 116.8 109.6 113.2 113.1 119.8 +[Hllb 127.2 122.9 121.0 106.2 107.4 +[&Ib 110.6 (b) Key Bond Lengths and Interatomic Distances (Average Values)o c4Xl 1.59 1.59 1.59 1.58 1.61 Cl-CIl 1.52 1.53 1.52 1.52 1.53 H~1~18 2.55 2.48 a Valueswere determined in CRYSTALS.19 b B[X1] is the dihedral angle defined by X(l)-C(l)C(2)-H(2) or X(l)C(l)-C(6)-H(6), X = H, C; e[&] by X(4)4(4)-C(3)-H(3), X(4)-C(4)4(5)-H(5), etc.; &[X111 by X(ll)-C(l)-C(2)-C(3) or X(ll)-C(l)-C(6)-C(5), etc.; see Figures 6 and 7.
pseudoequatorial and pseudoaxial substituents respectively, relative to the plane defined by one of the .rr-bonds in the dihydrobenzene ring. The torsion angles found in 2a and 2b are shown in Table V. Another interesting feature of these structures lies in the extremely long carbon-carbon single bond C(4)-C(7) which links the central carbon atom of the trityl substituent and the dihydroaromatic ring (N.B. the cis isomers 2 are thermally stable in solution (DMSO) up to ca. 400 K). For example, in 2b this bond is slightly over 1.61 A which compares with values of ca. 1.59 8,for the corresponding bond in the trans isomers2 (see Table V). These very long carbon-carbon single bonds are comparable with those found in cis-l,2-bis(methoxycarbonyl)-1,2-bis(p-nitropheny1)cyclobutane (1.606(3) A),20 4,4’-dihydro-1,3-bis(dimethylamino)-4,4’-biisoquinolylbis(perch1oate)(1.597(4) A),21 and 1.622-1.626 A found in some steroidal structuresaZ2The C-Ph bonds of the trityl groups of 2a and 2b lie within the range 1.53-1.58 A,values which are close to those previously reported for tetraphenylmethane (1.553
A=).
Comparison of the temperature factor data for the two structures 2a (room temperature) and 2b (143 K) as expected reveals significantly reduced motion in the latter structure. In both cases that for Hzl has a large amplitude along the y axis (> z > x ) and C110, CBI,and C132 show rather large thermal amplitudes in the x direction. About the cyclohexadiene ring Cq vibrates isotropically, whereas C1 wobbles somewhat less in the yz plane, and C7 is relatively immobile. We note that the temperature factors (20) Cam, P.; Finney, J. L.; Lindley, P. F.; De Titta, G. T. Acta Crystallogr. 1977, B33, 1022. (21)Boyd, G . V.; Hammerich, 0.; Lindley, P. F.; Mitchell, J. C.; Nicolaou, G . A.; Walton, A. R.J. Chem. Soc., Chem. Commun. 1985,885. (22) Smith,G. W. ActaCryutallogr.1975,831,522,526. vanSchalkwyk, T. G. D.;Kruger,G. J. Acta Crystallogr. 1974,B30,2261. Seealso: Beech, S. H.; Bauer, J. Y. J. Am. Chem. SOC.1942,64, 1142. (23) Robbins, A.; Jeffrey, G. A.; Chesick, J. P.; Donohue, J.; Cotton, F. A.; Frenz, B. A.; Murillo, C. A. Acta Crystallogr. 1975,B31,2395-2399.
Ulll are slightly negative for atoms CSZand C122 in the more highly absorbing bromo structure 2b. Figure 8 shows the similarities of the packing arrangements found in the structures of 2a and 2b. In both cases the molecules are arranged in columns perpendicular to the isolated 1-phenyl ring plane (Figure 8b,d). However, the need to accommodate the 4‘-bromo substituent in this ring in 2b leads to spaces in which the benzene of solvation is located. The channels running parallel with the molecular columns thus contain alternating benzene rings and bromo substituents. ‘HNMR SpectroscopicStudies. A detailed analysis of the nonaromatic regions of the ‘H NMR spectra of 2a, supported by selectivedecoupling and COSY experiments, leads to the values of the coupling about the cyclohexadiene ring shown in Table VI. The homoallylic coupling constant J1,d-Cis has a value typical for an interaction between two cis-protons placed in a pseudoaxial sense in a relatively planar cyclohexadiene ring.24 The vicinal and allylic couplings also support this interpretation, particularly when compared with the corresponding values for the trans isomers (see Table VI), these latter being regarded as consistent with an apparently planar ring geometry. The other important and unusual feature of the lH NMR spectra of both 2a and 2b lies in the unusually high-field aromatic resonance at ca. 6 6.3 ppm for 2a (6 6.15 ppm for 2b) with an integral corresponding to two protons. This was noted in the original study of these compounds3 but was not satisfactorily explained. In the spectrum of the parent hydrocarbon 2a this appears as a complexmultiplet which simplifies to half of an apparent AB quartet (J= 8.6 Hz) in the spectrum of 2b. Accordingly, this signal must be assigned to either the ortho or the meta protons in the isolated aryl substituent attached to C1 of the cyclohexadiene ring (see Figure 6). There is no significant change in this region of the spectrum when a solution of 2a in DMSO-ds is heated to ca. 403 K so that this feature does not result from slow rotation effects on the NMR time scale. However, when a solution of 2a in CDzClz is cooled to 203 K, this two-proton aromatic resonance is seen to move upfield by ca. 0.6 ppm. Slight changes are also observed in the chemical shifts of the peaks assigned to HI and H2 in both cis and trans isomers, though in these cases the resonance moves downfield (see Figure 9a). As the sample is cooled, considerable line-broadening occurs, but no coalescence phenomena associated with “freezing out” of the rotation of the isolated phenyl substituent about the C1-C11 bond are observed. Significant line broadening is also found in low temperature ( 50° (Figure ll).11931 A number of other examples to edge-to-face aromatic interactions in the solid state have been reported38 though in such cases the (C)H-n(centroid) separation is generally rather greater (usually ca. 2.80 A). However, Hamor, Jennings, and c o - w ~ r k e r have s ~ ~ observed a closer interaction (2.7 A) in the 2-isomer of N-[l-(1-naphthyl)ethylidenel-1-phenyl-2-propylamine in the solid state. This isomer is also favored in solution, and NMR experiments suggest that in this case the attractive edge-to-face interaction may be worth (Le., AH =) ca. 5 kJ mol-l, a value similar to that estimated for the stabilization resulting from edge-to-face association of two aromatic side chains in a protein.31 Stoddard has recently reported a very close (2.54 A) intermolecular interaction in the 1:l complex formed from a cyclobis(paraquat-p-phenylene)tetracation cyclophane and 1,5-dimetho~ynaphthalene.~ Acknowledgment. Thanks are due to Dr. H. S. Rzepa (Imperial College, University of London), Dr. J. K. M. Saunders (University of Cambridge), Professor M. J. Perkins (Royal Holloway and Bedford New College, University of London) and Dr. R. B. Mallion (The King’s School, Canterbury, Kent) for helpful comments. (37) Jorgensen, W. L.; Severance, D. L. J. Am. Chem. SOC.1990,112, 4768. (38) Other examples of aromatic edge-to-face interactions [H-tocentroid distances (r in Figure 11) or centroid-to-centroid ( d i nFigure 11) distance in brackets] includethe following. (i)Intramolecular: (a)Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. J. Chem. SOC., Chem. Commun. 1987,1070-1072 (r = 2.8A). (b) Ashton, P. R.; Chryetal, E. J. T.;Mathias, J. P.;Parry,K. P.;Slawin,A. M.Z.;Spencer,N.; Stoddart, J. F.; Williams,D. J. TetrahesronLett. 1987,50,6367-6370 (r = 2.81 A). (ii) Intermolecular: (c) Moody, G. J.; Owueu, R. K.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Thomas, J. D. R.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1987, 26, 890-892. (d) Ashton, P. R.; Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Stoddart, J. F.; Williams D. J. Angew. Chem., Int. Ed. Engl. 1988,27, 1550-1553 (d = 4.8,5.1 (e) Ashton, P. R.; Goodnow, T. T.; Kaifer, A. E.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1989,28,1396-1399. (0 Ortholand, J. Y.;Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1989,28, 1394-1395 (d = 5.17 A). See also: (0Jagg, P. N.; Kelly, P. F.; Rzepa, H.S.; Williams, D. J.; Woolins, J. D.; Wylie, W. J. Chem. SOC., Chem. Common. 1991, 942-944. (39) Hamor, T. A.; Jennings, W. B.; Proctor, L. D.; Tolley, M.S.;Boyd, D. R.; Mullan, T. J. Chem. SOC.,Perkin Trans. 2 1990,2530. (40) Reddington, M. V.; Slawin,A. M. Z.; Spencer, N.; Stoddart,J. F.; Vicent, C.; Williams,D. J. J.Chem. SOC., Chem. Commun. 1991,630-634 (r = 2.54 A; see Figure 11). (41) The coordinates can be obtained, on request, from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 lEZ, UK.
A).
